Maybe “fuel core” equals battery
Mid life servicing for a nuke boat (sub or carrier) is traditionally cutting out and replacing the reactor. (I am not a nuke engineer so dont flame me for details). What this article implies is a reactor fuel core that lasts twice as long as its predecessors. That seems like a significant tech leap, and I suppose its possible. Maybe we just dont hear about this since nuke electrical power plant construction in the US has been so taboo for the last 50 years.